Are you suggesting that by upping the NB Voltage I can raise the CPU REF higher?
What do you suggest for safe voltages to try for the NB?
But this is how I would overclock this chip "take a look at my sig".
I would drop the multi of the HT to x4 or x5 the CPU multi to x5 and the same for the ram.
Now from here I would increase the CPU REF/CPU FREQUENCY from 200MHz in 10MHz increments, while doing this stress the same as you would for a CPU using large FFT's.
Once you get to the point of a crash (the same as a CPU) then you increase the NB voltage to compensate for the increase of the CPU REF/CPU Frequency.
Once you start hitting the upper limit of 250MHz + then I would only increase at 5MHz a step and then increase NB voltage an increment at a time.
Once you have found the max of the CPU REF/ CPU Frequency and have it stable (like a CPU) then you increase the other multipliers of the CPU,HT,RAM,CPU/NB starting with the CPU first.
This may help out a bit.
But for a 4 GHz OC you will be looking at a CPU REF/ CPU Frequency of 285MHz-290MHz on a x14 multiplier.